EPOLL(7) Device and Network Interfaces EPOLL(7)


epoll - Linux-compatible I/O event notification facility


#include <sys/epoll.h>


epoll is a facility for efficient event-oriented I/O that has a similar
model to poll(2), but does not necessitate rescanning a set of file
descriptors to wait for an event. epoll is of Linux origins, and this
facility is designed to be binary-compatible with the Linux facility,
including the following interfaces:

o epoll_create(3C) creates an epoll instance, returning a file
descriptor. It contains a size argument which is meaningful
only in as much as it cannot be 0.

o epoll_create1(3C) also creates an epoll instance, but
eliminates the meaningless size argument -- replacing it
instead with a flags argument.

o epoll_ctl(3C) allows file descriptors to be added (via
EPOLL_CTL_ADD), deleted (via EPOLL_CTL_DEL) or modified (via
EPOLL_CTL_MOD) with respect to the epoll'd set of file

o epoll_wait(3C) fetches pending events for file descriptors
added via epoll_ctl(3C), blocking the caller if no such events
are pending.

o epoll_pwait(3C) operates in a similar manner to
epoll_wait(3C), but allows the caller to specify a signal mask
to be set atomically with respect to waiting for events.


The epoll facility is implemented for purposes of offering compatibility
to and portability of Linux-borne applications; native applications
should continue to prefer using event ports via the port_create(3C),
port_associate(3C) and port_getn(3C) interfaces. In particular, use of
epoll in a multithreaded environment is fraught with peril; even when
using EPOLLONESHOT for one-shot events, there are race conditions with
respect to close(2) that are unresolvable. (For more details, see the
aborted effort in Linux to resolve this via the proposed
EPOLL_CTL_DISABLE operation.) The event port facility -- like the BSD
kqueue facility that inspired it -- is designed to deal with such issues
via explicit event source dissociation.

While a best effort has been made to mimic the Linux semantics, there are
some semantics that are too peculiar or ill-conceived to merit
accommodation. In particular, the Linux epoll facility will -- by design
-- continue to generate events for closed file descriptors where/when the
underlying file description remains open. For example, if one were to
fork(2) and subsequently close an actively epoll'd file descriptor in the
parent, any events generated in the child on the implicitly duplicated
file descriptor will continue to be delivered to the parent -- despite
the fact that the parent itself no longer has any notion of the file
description! This epoll facility refuses to honor these semantics;
closing the EPOLL_CTL_ADD'd file descriptor will always result in no
further events being generated for that event description.


epoll_create(3C), epoll_create1(3C), epoll_ctl(3C), epoll_pwait(3C),
epoll_wait(3C), port_associate(3C), port_create(3C), port_dissociate(3C),
port_get(3C), pselect(3C)

May 16, 2020 EPOLL(7)